Gas turbine engines include a compressor for compressing ambient airflow which is then mixed with fuel in a combustor and ignited for generating hot combustion gases which flow downstream over rotor blades, stator vanes, and out an exhaust nozzle. These components over which flows the hot combustion gases must, therefore, be suitably cooled to provide a suitable useful life thereof, which cooling uses a portion of the compressed air itself bled from the compressor.
For example, a rotor blade or stator vane includes a hollow airfoil the outside of which is in contact with the combustion gases, and the inside of which is provided with cooling air for cooling the airfoil. Film cooling holes are typically provided through the wall of the airfoil for channeling the cooling air through the wall for discharge to the outside of the airfoil at a shallow angle relative to the flow direction of the combustion gases thereover to form a film cooling layer of air to protect the airfoil from the hot combustion gases and for cooling the airfoil. In order to prevent the combustion gases from flowing backwardly into the airfoil through the film holes, the pressure of the cooling air inside the airfoil is maintained at a greater level than the pressure of the combustion gases outside the airfoil to ensure only forward flow of the cooling air through the film holes and not backflow of the combustion gases therein. The ratio of the pressure inside the airfoil to outside the airfoil is conventionally known as the backflow margin which is suitably greater than 1.0 for preventing backflow.
The ratio of the product of the density and velocity of the film cooling air discharged through the film holes relative to the product of the density and velocity of the combustion gases into which the film cooling air is discharged is conventionally known as the film blowing ratio. The film blowing ratio, or mass flux ratio, of the injected film cooling air to the combustion gas flow is a common indicator for the effectiveness of film attachment. Values of the film blowing ratio greater than about 0.7 to 1.5, for example, indicate the tendency for the film cooling air to lift off the surface of the airfoil near the exit of the film cooling hole, which is conventionally known as blow-off. Effective film cooling requires that the film cooling air be injected in a manner which allows the cooling air to adhere to the airfoil outside surface, with as little mixing as possible with the hotter combustion gases.
One conventionally known method to aid in obtaining effective film cooling is to inject the cooling air at a shallow angle relative to the outside surface to reduce blow-off tendency. The blow-off of film cooling air increases mixing with the hotter gases to varying extents, depending upon the severity of the blow-off. This results in a decrease in the effectiveness of the film cooling air and, therefore, decreases the performance efficiency of the cooling air which, in turn, reduces the overall efficiency of the gas turbine engine.
Another common indicator of film effectiveness is the film coverage. The coverage is generally known as the fractional amount of the airfoil outside surface which is thought to have film injected over it, at the exit of a row of film cooling holes. An increased coverage generally, but not necessarily, means an increased film effectiveness. The maximum coverage which may be obtained for a single configuration of film cooling is 1.0.
In order to reduce the film blowing ratio, it is known to provide tapered film cooling holes which reduce the velocity of the film cooling air as it flows therethrough by the conventionally known diffusion process for improving the effectiveness of the film cooling air discharged from the hole. It is also conventionally known to provide a longitudinally extending slot in the airfoil wall which is disposed perpendicularly relative to the direction of the combustion gases, with the slot being fed by a plurality of longitudinally spaced apart film cooling metering holes. The slot provides a plenum of increased area relative to the collective area of the metering holes which, therefore, reduces the velocity of the film cooling air therein by diffusion prior to discharge from the slot along the wall outer surface. In addition, the provision of a slot and the effective diffusion of cooling air within this slot serves to increase the film coverage as the cooling air exits onto the airfoil outside surface.
Various embodiments of film cooling holes feeding diffusion holes or slots are known and have varying degrees of complexity, and, therefore, require relatively complex fabrication processes which increases manufacuting costs which can be substantial for mass produced components such as turbine vanes and blades. Furthermore, it is desirable to have shallow injection angles down to about 15.degree., but such small angles formed at the film cooling holes reduces the strength of the hole at this location and requires more precise manufacturing to obtain.